Since Arris router has a coaxial cable, does your modem support coaxial cable access? If yes, you may try to remove Arris router to reconnect your modem and router. Tips: you may also contact your...

Is there any note it can't be downgraded shown under FW160125? What's the error message when you try to downgrade? Generally speaking, you can't update to other firmware if FW150507 is special one...

If you sign in to ISP portal once, does it still ask again next time?

you can confirm whether it's still under warranty. If yes, contact tp-link support via email to get a replacement.

hey, in that case, advise you to email/contact tp-link support directly for some help.

no, different hardware versions can't be updated. Therefore, you can't update from V2 to V4.